<<What are the plans of the remaining two tenants? If they are going to continue the lease, they have a legal obligation to find another tenant to share the space with them.

No, they don't. They have an obligation to pay the rent, along with the son. The landlord won't care if a bedroom is empty >>

It's a principle of contract law that a person has an obligation to mitigate their damages.

In cases involving tenants who move out, that means that the remaining roommates or the landlord have an obligation to seek another tenant to minimize any losses created by someone moving out before a lease expires.

If they fail to do that, they will probably lose their ability to collect damages for someone who leaves early.

Think about it ---

Suppose someone signs a lease for a year and moves out after a month. Should a landlord be able to keep the place vacant and charge the tenant for a twelve months rent?

The answer is --- no. the landlord has an obligation to re rent the apartment as quickly as he reasonably can, and the obligation of the tenant to pay rent under the lease goes away when a new tenant starts paying rent.